(1) Field of the Invention
The invention relates to a process for extruding plastic compositions, in particular polymer melts and mixtures of polymer melts, above all thermoplastics and elastomers, particularly preferably polycarbonate and polycarbonate blends, also with the incorporation of other substances such as for example solids, liquids, gases or other polymers or other polymer blends with improved optical characteristics, with the assistance of a multi-screw extruder with specific screw geometries.
2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Extrusion is a known process in the production, compounding and processing of polymers. Extrusion is here and hereinafter taken to mean the treatment of a substance or substance mixture in a co-rotating twin- or multi-screw extruder, as is comprehensively described in [1] ([1]=Kohlgrüber. Der gleichläufige Doppelschneckenextruder [The co-rotating twin-screw extruder], Hanser Verlag Munich 2007).
The treatment of plastic compositions during extrusion includes one or more of the operations: conveying, melting, dispersion, mixing, expulsion of liquid constituents, degassing and pressure build-up.
In polymer production, extrusion serves, for example, to remove volatile constituents such as monomers and residual solvents from the polymer ([1], pages 192 to 212), to carry out polyaddition and polycondensation reactions and optionally to melt and convert polymers and optionally to mix additives with the polymer.
During polymer compounding, extrusion is above all used to produce mixtures of polymers with additives and auxiliaries and reinforcing materials and colors and to produce mixtures of different polymers which differ, for example, in chemical composition, molecular weight or molecular structure (see for example [1], pages 59 to 93). Compounding involves the conversion of a polymer into a finished plastics molding composition (or compound) using plastics raw materials, which are conventionally melted, and adding and incorporating and mixing fillers and/or reinforcing materials, plasticizers, bonding agents, slip agents, stabilizers, colors etc. with the polymer. Compounding often also includes the removal of volatile constituents such as for example air and water. Compounding may also a chemical reaction such as for example grafting, modification of functional groups or molecular weight modifications by deliberately increasing or decreasing molecular weight.
As is generally known and described, for example, in [1] on pages 169 to 190, mixing may be differentiated into distributive and dispersive mixing. Distributive mixing is taken to mean the uniform distribution of various components in a given volume. Distributive mixing occurs, for example, when similar polymers are mixed. In dispersive mixing, solid particles, fluid droplets or gas bubbles are firstly subdivided. Subdivision entails applying sufficiently large shear forces in order, for example, to overcome the surface tension at the interface between the polymer melt and an additive. Mixing is always understood below to mean distributive and dispersive mixing.
Melt conveying and pressure build-up are described on pages 73 et seq. of publication [1]. The melt conveying zones serve to transport the product from one processing zone to the next and to draw in fillers. Melt conveying zones are generally partially filled, such as for example during the transport of the product from one processing zone to the next, during degassing and in holding zones.
During polymer processing, the polymers are preferably converted into the form of a semi-finished product, a ready-to-use product or a component. Processing may produce, for example, by injection molding, extrusion, film blowing, calendering or spinning. Processing may also involve mixing polymers with fillers and auxiliary substances and additives as well as chemical modifications such as for example vulcanization.
As a person skilled in the art is aware, polymer extrusion is advantageously performed on extruders with two or optionally more screws.
Co-rotating twin- or optionally multi-screw extruders, the rotors of which are fully self-wiping, have long been known (DE 862 668). Extruders which are based on the principle of fully self-wiping profiles have been put to many different uses in polymer production, compounding and processing. Such extruders are known to have a good mixing action, a good degassing action and a good action for melting polymers. They offer advantages in the quality of the products produced therewith because polymer melts adhere to surfaces and degrade over time at conventional processing temperatures, which is prevented by the self-cleaning action of fully self-wiping screws. Rules for producing fully self-wiping screw profiles were stated, for example, in Klemens Kohlgrüber: Der gleichläufige Doppelschneckenextruder [The co-rotating twin-screw extruder], Hanser Verlag Munich 2007, p. 96 et seq. [1]. The design of single-, double- and triple-flighted profiles is described therein.
It is known to a person skilled in the art that in the region of the screw tips a particularly large amount of energy is dissipated in the melt, which leads locally to severe overheating in the product. This is explained, for example, in [1] on pages 160 et seq. This local overheating may result in harm to the product such as for example a change in odor, color, chemical composition or molecular weight or in the formation of non-uniformities in the product such as gel particles or specks. A large tip angle, in particular, is harmful in this respect.
Modern twin-screw extruders have a building-block system, in which various screw elements may be mounted on a core shaft. In this way, a person skilled in the art may adapt the twin-screw extruder to the particular task in hand. As a rule, screw elements with double- and triple-flighted profiles are used today, since single-flighted screw profiles have an excessively high energy input due to their large tip angle.
According to the prior art [1] (see, for example, page 101), the geometry of the fully self-wiping screw elements is defined via the following independent variables: the number of flights Z, centreline distance A and barrel diameter (corresponding to the diameter DE of the fully self-wiping profile). The number of flights here is the number of circular-arcs of each element which wipes the outer wall. The angle of any such circular arc, relative to the centre of rotation, is termed the tip angle KW0. In the region defined by the tip angle, the outer radius of the profile is equal to the barrel radius. According to the prior art, KW0 is not an independent variable that can be adjusted appropriately for the task in hand, but instead is a result of Eq. 1, being
                              KW          ⁢                                          ⁢          0                =                              π            Z                    -                      2            ⁢                                                  ⁢                          arccos              ⁡                              (                                  A                  DE                                )                                                                        (                  Eq          .                                          ⁢          1                )            where KW0 is the tip angle of the fully self-wiping profile in radians and π is the constant (π≈3.14159) that relates the circumference of a circle to its radius. The total of the tip angles across both elements of a closely intermeshing pair of elements SKW0 is necessarily
                              SKW          ⁢                                          ⁢          0                =                              2            ⁢            π                    -                      4            ⁢            Z            ⁢                                                  ⁢                          arccos              ⁡                              (                                  A                  DE                                )                                                                        (                  Eq          .                                          ⁢          2                )            
The person skilled in the art is aware that directly self-wiping screw profiles cannot be inserted directly into a twin-screw extruder, but rather there have to be some clearances between the screw elements and the barrel and/or between the screw elements themselves. The person skilled in the art uses known methods, such as those described by way of example in [1], to obtain the geometric data for the actual screw geometries, on the basis of the contour of fully self-wiping screws. Pages 28 ff. in [1] describe various possible strategies for conveying elements. When the longitudinal section offsets or three-dimensional offsets stated in that reference are used, the tip angles KWA0 of the actual screws become smaller, as described by way of example in [1], p. 100, with respect to the angle KW0. In particular, large clearance between the screws leads to reduced KWA0. However, large clearance between the screws, reducing the tip angle, is disadvantageous, because it diminishes the amount of mutual self-cleaning effect provided by the screws, and long residence times occur at the surface of the screw elements, leading to local product degradation and therefore to impairment of product quality. The person skilled in the art is also aware that enlargement of the clearances impairs the effectiveness of screw elements in relation to conveying action and pressure increase, and successful completion of any given processing task therefore requires that excessive clearances be avoided.